What's the best/simplest way to run heaters to a 6SN7 and a

dehughes · Post by **dehughes** » Wed Dec 12, 2007 5:48 am

...12AX7? I'm looking to run both in the same amp, and while I'm familiar with the standard way to wire up 12A*7 heaters, I'm having a tough time conceptualizing how one would run heaters to both 12AX7 and 6SN7 tubes so that they both operate on their required heater voltages.

Many thanks in advance...

Johnhenry · Post by **Johnhenry** » Wed Dec 12, 2007 6:12 am

it has 6.3 volt heater's, just like your 12AX7 and you will find pin's 7 & 8 are your heater's on the 6SN7.
John

tele_player · Post by **tele_player** » Wed Dec 12, 2007 6:14 am

All guitar amps I've seen run 12ax7 on 6.3v heater, just like a 6SN7.

stoo · Post by **stoo** » Wed Dec 12, 2007 6:44 am

The 12 volt heaters in a 12ax7 are really two 6.3 volt heaters in series. One side of the heater winding goes to the junction of those 2 filaments (pin9) and the other winding goes to both of the outer ends (pins 4 &5)
On a 6SN7 there's just two heater connections.
Stew

dehughes · Post by **dehughes** » Thu Dec 13, 2007 12:58 am

Okay...so, if the 6SN7 was the "last" tube in the heater wire chain, I'd just connect one heater wire to 7 and the other to pin 8 of the 6SN7? Is it really that simple? Did I conceptualize it to be much more difficult than it really is?

tele_player · Post by **tele_player** » Thu Dec 13, 2007 1:11 am

Yes, and it's just as simple if the 6SN7 isn't last in the chain.

dehughes · Post by **dehughes** » Thu Dec 13, 2007 4:25 am

tele_player wrote:Yes, and it's just as simple if the 6SN7 isn't last in the chain.

Sweet. Thanks! I don't know why I had it so convoluted in my mind...

dehughes · Post by **dehughes** » Wed Mar 11, 2009 5:22 am

So, I noticed that on the Komet Constellation the 6SN7 and the 1st 12AX7 run on a DC heater. In looking at the Boozhound Laboratories site he uses DC heaters on his octals. Should I worry about that in my build (same preamp tube compliment as the Constellation), or just leave the heaters AC and see?

paulster · Post by **paulster** » Wed Mar 11, 2009 8:28 am

Octals can be prone to heater-induced hum on AC which is why lots of builders use DC for these tubes.

If you're going to supply DC to the octal then you might as well supply DC to the first noval gain stage as well since any noise induced there will go through successive amplification stages. That seems to be the rationale that lots of builders use.

Getting quiet DC is a bit of a pain so you might want to try it on AC and see how you go, but accept that you may need to add it a rectifier and sufficient filtering to get quiet DC if the AC doesn't cut it.

Phil_S · Post by **Phil_S** » Wed Mar 11, 2009 1:50 pm

Last week, I finished this all octal amp:
http://home.comcast.net/~psymonds/6SN7amp.htm
I'm not making any excuses for the poor guitar playing and note, if you choose to suffer through the sound samples, the clipping you hear is a result of the recording process, not from the amp itself.

Heater hum is not a problem. The amp is quiet at idle with no instrument plugged in. Just follow the time tested "rules" of lead dress and you should be fine.

dehughes · Post by **dehughes** » Wed Mar 11, 2009 8:39 pm

Thanks Paulster. I'll wire it up now and see what I hear...

Phil_S, cool build. Is there any rationale to running separate heater wires to the preamp tubes and power amp tubes (instead of stringing them together)?

Phil_S · Post by **Phil_S** » Wed Mar 11, 2009 9:25 pm

dehughes wrote:Phil_S, cool build. Is there any rationale to running separate heater wires to the preamp tubes and power amp tubes (instead of stringing them together)?

Thanks. It took 3 different power transformers to get this one right. The first one had two wimpy 6.3v windings, so I divided the tubes. I thought about it and decided the path of least resistance would prevail, rather than rewire.

dehughes · Post by **dehughes** » Sat Mar 21, 2009 5:54 am

So, does it matter which filament wire goes to which pin on the 6SN7, in terms of how the wires are connected to the 12AX7? I didn't know if there was a "backwards" way to do it (like pin 9 on a 12AX7 connects to pin 7 on the 6SN7, etc...).

Phil_S · Post by **Phil_S** » Sat Mar 21, 2009 2:16 pm

Some people are downright anal about wiring up the heaters so that the same side of the winding goes consistently to the same pins on the tubes. By that, I mean you use, let's say, one white and one green wire, You always put the white wire on pin 9 and the green on pins 4&5. I don't buy into this. I just can't understand why it could matter. Others may disagree.

Seeing as how I don't think it matters, I typically use 2 green wires and just string them across the filaments in any convenient configuration. I've never had a problem and my amps don't hum. So, just wire it up. My sense is that you are overthinking this. IMO, there is no "wrong" or "backwards".

Heaters are 7 and 8 on a 6SN7. The only wrong way to do it is to attach the filament supply to the wrong pins. Go for it!

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